Infectious Bronchitis
Etiology
Infectious bronchitis (IB) is a highly contagious respiratory disease capable of producing a range of clinical signs in chickens. The disease can vary from asymptomatic to involvement of the respiratory, renal, and reproductive organs. The disease is caused by the infectious bronchitis virus (IBV), a single stranded, RNA, positive sense, enveloped virus and a member of the family Coronaviridae, genus Coronavirus. There are many strains of IBV with considerable variation in the virulence.
Host Range
The chicken is the only species known to be naturally susceptible to IBV. Susceptibility to the virus varies by breed and age. Young chicks develop the most severe respiratory forms of the infection, whereas older birds are most susceptible to reproductive lesions and drops in egg production. Although all ages of chickens are susceptible, as birds mature, they become more resistant to these effects. Environmental factors and the presence of concurrent infections often play a significant role in the severity of clinical signs. IBV does not appear to pose a threat to human health.
Epidemiology
The IB virus is shed in the nasal excretions and feces of infected birds. The virus is highly infectious and typically spreads quickly from bird to bird, via direct and indirect contact with contaminated feed, water, equipment, and other infected birds. In some birds, internal organs become persistently infected, resulting in intermittent shedding of the virus. These carriers increase the possibility of flock-to-flock spread of the virus via unknowingly contaminated personnel.
The incubation time for the IBV varies from 18-36 hours. Morbidity is 100%, however mortality varies greatly depending on the virulence of the strain, host age, immune status, and the presence of physiologic stress and concurrent infections. Mortality is usually less than 5%, however secondary infections with Eschericia coli and Mycoplasma can significantly increase the mortality rate.
The IB virus is distributed worldwide, with different serotypes found in different regions of the world. Despite vaccination efforts, outbreaks of IB occur frequently because vaccines against one serotype do not cross-protect against a different serotype.
Clinical Signs
The clinical signs of the infection vary and depend on the age of the bird, host immune status, and virulence of the virus. In chicks, depression, ruffled feathers, and huddling near heat sources are common early signs. Within 24 hours, respiratory signs are typically observed and may include coughing, sneezing, nasal discharge, tracheal rales, and gasping. Ocular discharge (epiphora) may be present and the sinuses may become swollen. Feed intake may drop and subsequent weight loss may be observed. Chicks, less than 2-weeks of age may suffer damage to the oviduct, resulting in permanent impairment to their egg-laying capacity. Some highly virulent strains have also been associated with facial edema and airsacculitis.
In birds older than 6 weeks of age, the signs are similar but often less severe. In adults, respiratory signs may be so subtle as to be observable only at night when the birds are normally quiet. In layers, egg production may drop by up to 50% (depending on virus strain and period of lay) and eggs are often misshapen, soft-shelled, and contain watery albumen. Birds that recover from the infection may never return to pre-infection egg laying levels.
Some strains of IBV are nephropathogenic. Birds generally recover from early respiratory signs only to later develop diarrhea, sometimes with fatal secondary urolithiasis.
Post-mortem Lesions
The trachea, nasal passages, and sinuses may be edematous and typically have serous, catarrhal, or caseous exudate present on post-mortem examination. A caseous plug may occlude the trachea or bronchi in dead chicks. The lungs may show evidence of pneumonia. The air sac membranes may be cloudy and caseous yellow exudate may be present. The kidneys may be swollen and pale, due to interstitial nephritis and necrosis, and white urates may be present in increased amounts in the distended tubules and ureters. The oviduct may be occluded and hypoglandular. Egg yolk peritonitis may be found within the coelomic cavity, secondary to ruptured follicles.
Differential Diagnosis
The clinical signs of IB may resemble other acute respiratory diseases in chickens, such as Newcastle disease, laryngotracheitis, avian influenza and infectious coryza. Laboratory testing is necessary to differentiate these infections. In IB, allantoic fluid from inoculated embryos does not hemagglutinate erythrocytes. For Newcastle disease virus, and avian influenza virus, allantoic fluid from inoculated embryos will be positive for hemagglutination.
Diagnosis
Clinical history, gross lesions, seroconversion and molecular tests for antigen and viral nucleic acid detection are commonly used to diagnose infectious bronchitis.
Diagnostic samples should be collected as soon as clinical signs are observed. Tracheal swabs should be collected from 5-10 birds per affected flock. Additionally, paired serum samples should be collected. Fresh tissue samples should be taken from the lung, kidney, oviduct, and cecal tonsils using aseptic technique. Swabs and tissue samples should be frozen and all samples shipped to the laboratory.
In the laboratory, swab tube broth or tissue homogenates are used to inoculate 9 to 11 day old embryonated eggs. After several days, chorioallantoic fluid is harvested and should produce a negative hemagglutination reaction with chicken red blood cells. Curling, stunting and death of embryos can be seen in IBV positive embryos. Confirmation of IBV, and its serotypes, can be performed by various antibody detection methods including: virus neutralization (VN), immunoflourescent antibody assay (IFA), antigen-capture enzyme-linked immunosorbent assay (AC-ELISA), and monoclonal antibodies. However, only the VN test and some monoclonal antibodies can distinguish between different serotypes. Reverse transcriptase-polymerase chain reaction (RT-PCR), followed by restriction fragment length polymorphism (RFLP), and/or sequencing of the S1 gene are commonly used to identify IBV types. The immunoflourescent antibody assay (IFA) or electron microscopy can be used on tracheal samples for rapid diagnosis but those tests do not distinguish between different IBV types.
Seroconversion, and specifically a rise in IBV antibody titer indicating IBV infection, is determined by ELISA, VN test, modified hemagglutination inhibition (HI), immunofluorescence, and immunodiffusion tests.
Prevention and Control
Prevention and control in the field is difficult using common sanitary and biosecurity measures because IBV is highly infectious. Good cleaning and disinfection practices, during the downtime period, may help to prevent recurrent infections in problem farms. Farm complexes with multiple ages, such as the ones that have table egg layers, are a common source of infection. In these types of flocks, it is very common to see drops in egg production due to chronic disease.
Infectious bronchitis is extremely difficult to control because different serotypes of the virus do not cross-protect. Attenuated live and inactivated vaccines are used for infectious bronchitis immunization. Attenuated live vaccines delivered in the drinking water or by spray, are commonly used in broilers and for the initial vaccination of laying birds. Inactivated oil-emulsion vaccines are used in breeders and layers before the beginning of the production cycle. The administration is performed either by intramuscular or subcutaneous injection. These types of vaccines reduce the incidence of viral replication in the respiratory tract and may limit the spread and transmission to other susceptible birds. Broilers are commonly vaccinated with a live vaccine in the hatchery, followed by a second vaccination of the same or different serotype between 10 and 18 days of age.
No specific treatment exists for infectious bronchitis. The use of antibacterials against secondary opportunistic bacterial infections in the first stages of the disease may reduce the incidence of severe multifactorial respiratory disease, associated with poor conversion performance and increased mortality.
Selected References
- Cavanagh, D. and J. Gelb. 2008. Infectious bronchitis. In Diseases of Poultry, 12th ed. Y.M. Saif. et al. (ed.). Blackwell Publishing, Ames, Iowa.
- Cavanagh, D. 2007. Coronavirus avian infectious bronchitis virus. Vet Res Mar-Apr;38(2):281-97.2007. Review.
- Charlton, B. R. (ed). 2006. Avian Disease Manual, 6th ed. American Association of Avian Pathologists (AAAP), 953 College Station Road, Athens, Georgia 30602-4875.
- Fabricant, J. 1998. The early story of infectious bronchitis. Avian Diseases 42:648-650.
- Gelb, J. and M.W. Jackwood. 2008. Infectious bronchitis. In A Laboratory Manual for the Isolation and Identification of Avian Pathogens, 5th edition. L. Dufour-Zavala Louise et al. (ed.) OmniPress, Inc., Madison, Wisconsin.
- Ignjatovi?, J. and S. Sapats. 2000. Avian infectious bronchitis virus. Rev Sci Tech Aug;19(2):493-508. Review.
- World Organization for Animal Health (OIE) website. 2008. www.oie.int
Thank you to the following individuals for reviewing these materials:
Mark Jackwood
Jaime Ruiz
Jose Bruzual
